Returning dead BeagleBone Blacks for repair

I have a stack of seven BeagleBone Blacks that have served me well, but in the hands of inexperienced students they have made the ultimate sacrifice. That is, they are dead.

How do I go about returning them to get repaired (if possible)?

–Mark is the return process. I'm guessing
that since these reflect mostly user error that they wouldn't be
replaced for free.